With advancing age, humans exhibit characteristic changes in temporal distribution of sleep and wakefulness. In elderly humans, daytime naps are common, nocturnal sleep is poorly consolidated, and the sleep period often begins and ends earlier than in young adults. These changes represent a reduction in amplitude of the circadian rhythm of sleep/wake that reflects deterioration of the circadian timing system in the aged. In mammals, the circadian timing system contains a biological "clock", located in the suprachiasmatic nucleus (SCN) of the hypothalamus, that regulates physiological and behavioral rhythmicity. This proposal will investigate age-related changes in the molecular mechanisms involved in synchronizing circadian rhythms to the environment. The major underlying hypothesis is that the increasing inability of the biological clock to entrain circadian rhythms with age is related to a deficit in the molecular mechanism underlying the transduction and processing of environmental (e.g., photic) information by the circadian system. Photic-induced expression of a number of cellular immediate early genes (IEGs) will be examined in the SCN and intergeniculate leaflet (IGL) of young and old animals by in situ hybridization histochemistry to determine whether functional deficits exist in the retinohypothalamic, retinogeniculate and/or geniculohypothalamic pathways. To determine whether deficits exist in transduction at the membrane receptor level in the aged animal, the efficacy of IEG induction will be evaluated in young and aged animals when the SCN is stimulated directly with neurotransmitters known to induce phase-shifts of the circadian system (e.g., neuropeptide Y, serotonergic agents). To determine whether altered regulation of putative "target" genes regulated by the IEGs occurs in the aged SCN, the levels of specific SCN neuropeptide mRNAs (somatostatin, VIP, vasopressin, GRP, neurotensin) induced by photic stimulation will be evaluated in young and old animals by Northern analysis. Based on the results of the above-mentioned experiments, the neurochemical identity of cells expressing IEGs in the SCN will be identified using double labelling techniques involving a combination of isotopic and non-isotopic detection. Lastly, the circadian rhythm of IEG mRNA expression in various brain regions outside the SCN will be investigated and alterations in this rhythm that may accompany aging. Along with other projects in this Program Project proposal, the experiments described herein should determine whether age-related changes in sleep and wakefulness result from either a deterioration of the circadian pacemaker itself or the coupling between the pacemaker and the driven rhythm (or both).